Spectrometers are generally measuring or analysis devices for the spectroscopic investigation of a sample. Optical spectrometers use light for the spectroscopic investigation. Spectroscopic investigations with photon density waves permit the separate absolute determination of absorption and scattering properties of strongly scattering samples, which is not possible with conventional methods of light scattering or of absorption spectroscopy. The term “photon density waves” means the time- and space-dependent propagation of light in a strongly scattering medium, which is characterized by the number of photons per volume, i.e. the photon density.
The description of photon propagation is based on investigations of the transport of neutrons, as both photons and neutrons are uncharged and, apart from scattering processes, move freely of external forces. However, the description is simplified considerably for photons because, in contrast to neutrons, they all have a constant, equal velocity. Starting from the radiation transfer equation, in which all contributions that lead to a change in light intensity are balanced, generally an expansion to a series of spherical harmonics is used, in order to obtain a simple description of photon density. Similarly to particle transport, an optical diffusion coefficient appears as a characteristic quantity, and is described by absorption and scattering properties of the medium.
As the propagation of light is affected characteristically by absorption and scattering, analysis of photon density waves can be used for determining these optical properties. The known experimental techniques for utilizing photon density waves can be classified on the basis of the time dependence of the incident light used in each case. On the one hand, for investigations in the time domain, a very short light pulse is irradiated into the medium, and then the space-dependent time dependence of light propagation is monitored. On the other hand, for experiments in the frequency domain, light sources are used whose intensity can be modulated sinusoidally. Furthermore, investigations can also be carried out with time-constant light sources.
Spectroscopy based on photon density waves is suitable in particular for investigating strongly scattering media with relatively low absorption. On the one hand high scattering permits light propagation to be described as random particle transport. On the other hand, when there is strong absorption the photon density decreases so much that detection is difficult. Investigations by means of photon density waves are therefore often carried out in a spectral range from 600 to 900 nm, where many materials only display moderate absorption. Scattering is caused by local variation of the refractive index and is especially pronounced when the scale of magnitude of the variation corresponds to the wavelength of the light and the difference in refractive index is large. In particular, therefore, dispersions of materials with different refractive index, whose disperse phase consists of particles with a diameter of over 50 nm, display pronounced scattering in the visible region of the spectrum.
In a spectrometer for investigating a sample by means of photon density waves, usually excitation light, which is produced by a light source, for example a laser, is coupled onto the sample to be investigated by a coupling-in device. This takes place for example using an optical waveguide in the coupling-in device. A coupling-out device is then used for coupling-out measurement light arising in the sample to be investigated owing to the incident excitation light and feeding it to a detection device. The coupling-out device can also be formed using one or more optical waveguides, into which the measurement light is coupled-in, in order to guide it to the detection device. Based on considerations of physical models, the detected measurement light can be evaluated, in order to determine optical properties of the sample to be investigated.
The coupling-in of the excitation light and the coupling-out of the measurement light require particular attention, in order to receive measurement light signals that can actually be evaluated.